The present invention relates to traction modifying differentials, and more particularly, to such differentials of the type in which the differential action may be retarded, and possibly even prevented (“locked”), in response to some sort of an input, for example, a mechanical input or an electrical input signal.
Furthermore, the present invention relates to engagement sensing mechanisms and systems of the type which may be utilized to sense a change-of-state within a traction modifying differential, for example, a change between an unlocked condition and a locked condition.
Traction modifying differentials of the type to which the present invention relates typically include a gear case defining a gear chamber, and disposed therein, a differential gear set including at least one input pinion gear, and a pair of output side gears. The present invention will be described in connection with a differential of the bevel gear type, although those skilled in the art will understand that the invention is not so limited, and could be utilized in connection with differentials having other gearing types, such as helical or planetary. Typically, a clutch pack is disposed between at least one of the side gears and an adjacent surface of the gear case, such that the clutch pack or locking mechanism is operable to limit relative rotation between the gear case and the one side gear. In most differentials of the type described, engaging the clutch pack or locking mechanism (to retard differentiation) is achieved by one of several different approaches.
In one approach, a “locking differential” of the type illustrated and described in U.S. Pat. No. Re 28,004, assigned to the assignee of the present invention and incorporated herein by reference, the clutch pack is normally disengaged. When one of the wheels begins to spin out, relative to the other wheel, a speed sensitive mechanism senses the speed differential between the wheels, and by means of a cam and ramp mechanism, locks the clutch pack solid. In the incorporated patent, the speed sensitive mechanism comprises a fly-weight mechanism, the output of which comprises the mechanical “input”, in response to which the differential gearing is locked.
U.S. Pat. No. 5,019,021, also assigned to the assignee of the present invention and incorporated herein by reference, illustrates another approach to retarding differentiation. This patent illustrates and describes a “limited slip differential” in which the loading on the clutch pack may be varied in response to an external electrical input signal, thus varying the amount of slip within the clutch pack. Therefore, the amount of bias torque transmitted from one side gear to the other is also varied in response to changes in the external electric input signal. As is well known to those skilled in the art, in a limited slip differential, there is typically a certain amount of “slip” or speed differential, between the two side gears whenever the vehicle encounters less than optimum traction conditions. In the above-incorporated patent, the “input” to the differential is the electrical input signal, but within the differential, there is another “input” which is the axial movement of one of the plates of a ball ramp actuator, the axial movement of which varies the loading on the clutch pack in a manner which is now generally well known to those skilled in the art.
Finally, in U.S. Pat. No. 6,551,209, also assigned to the assignee of the present invention and incorporated herein by reference, there is illustrated a different approach to a “locking differential”. In the above-incorporated patent, there is illustrated and described a locking differential in which there is no friction-type clutch pack, but instead, a mechanical locking arrangement. In the differential of the cited '209 patent, there is a ball ramp actuator which is able, in response to an electrical input signal, to move a series of pins into mating openings in the differential side gear, thus locking the side gear relative to the differential gear case. For purposes of the present invention, the movement of the pins, toward or away from the side gear, to achieve either a locked condition or an unlocked condition, respectively, is also considered an “input” in regard to a means for limiting rotation of an output gear relative to a gear case in a differential.
Thus, it may be seen, from a review of the above-described types of limited slip and locking differentials, that there are a number of different mechanisms known to those skilled in the art which are commonly used to limit (retard), or lock, the relative rotation between a differential gear case and one of the output side gears. However, it should be noted that most of the known, prior art limited slip and locking differential arrangements, and especially those which have been commercialized by the assignee of the present invention, have in common the presence of some sort of member which moves axially, in connection with the operation of the mechanism which achieves the slip limiting or locking function within the differential.
More recently, an increasing percentage of vehicles (especially passenger cars and light trucks) are incorporating some sort of stability, or traction, or safety system into the drive train. Examples of such systems would include a traction control system (TCS), an anti-skid braking system (ABS), and an electronic stability program (ESP). It is quite common, and desirable, for such systems to include some sort of traction modifying device, and preferably, an electrically-actuated limited slip or locking differential. In order for these types of systems to operate most effectively and safely, it is important for the control logic of the system to receive some sort of feedback signal from the differential, whereby the control logic can know, at any given instant, whether the differential is in an actuated (locked) condition, or in an unactuated (unlocked) condition.
Unfortunately, sensing the occurrence of a locked condition (or an unlocked condition) in a locking differential, or sensing an increasing clutch engagement (or a decreasing clutch engagement) in a limited slip differential involves sensing something such as the axial movement of a member within a differential gear case which, typically, is rotating within a stationary outer housing. One seemingly obvious way of mounting a sensor on a rotating differential case is to fix the sensor to the exterior of the case, and transmit the generated electrical signal from the differential to the vehicle microprocessor by means of slip rings. Unfortunately, such an arrangement is typically not feasible. For most differential installations, nothing can be attached to the exterior of (the outer diameter of) the differential case (or extend radially outward therefrom), because, in the axle assembly plant, it must be possible to slide the ring gear over the case outer diameter, and bolt the ring gear to the case flange.
Another hindrance encountered by those skilled in the art, in attempting to develop arrangements for sensing the “change-of-state” in a limited slip or locking differential is the fact that the sensing system utilized needs to be able to survive and operate effectively in a fairly severe environment. For example, the sensing mechanism and the overall system need to be able to operate predictably over a broad temperature range (e.g., from about −40 degrees Celsius to about 190 degrees Celsius). Also the sensing mechanism must be able to operate while submersed in a petrochemical-based lubricant, without any adverse effect upon the accuracy of the sensing system “output” signal, indicating the current state of the differential.